specified register. These write and read transmissions

can be joined using a repeated START even though the

MAX6621 7-bit slave address needs to be present pre-

ceding the R/W bits.

Start and Stop Conditions

Both SCL and SDA remain high when the interface is

not busy. A master signals the beginning of a transmis-

sion with a START (S) condition by transitioning SDA

from high to low while SCL is high. When the master

has finished communicating with the slave, it issues a

STOP (P) condition by transitioning SDA from low to

high while SCL is high. The bus is then free for another

transmission (Figure 6).

Data Transfer and Acknowledge

One data bit is transferred during each clock pulse.

The data on SDA must remain stable while SCL is high

(Figure 7).

The acknowledge bit is a clocked 9th bit that the recipi-

ent uses to handshake receipt of each byte of data

(Figure 8). Thus, each byte transferred effectively

requires 9 bits. The master generates the 9th clock

pulse, and the recipient pulls down SDA during the

acknowledge clock pulse so that the SDA line is stable

low during the high period of the clock pulse. When the

master is transmitting to the MAX6621, the MAX6621

generates the acknowledge bit because the MAX6621

is the recipient. When the MAX6621 is transmitting to

the master, the master generates the acknowledge bit

because the master is the recipient.

Slave Address

The MAX6621 has a 7-bit long slave address (Figure 9).

The 8th bit following the 7-bit slave address is the R/W

bit. The R/W bit is low for a write command and high for

a read command.

14

______________________________________________________________________________________

SDA

SCL

START

CONDITION

STOP

CONDITION

S

P

Figure 6. Start and Stop Conditions

SDA

SCL

DATA LINE STABLE;

DATA VALID

CHANGE OF DATA

ALLOWED

Figure 7. Bit Transfer

SCL

SDA BY

TRANSMITTER

CLOCK PULSE

FOR ACKNOWLEDGEMENT

START

CONDITION

SDA BY

RECEIVER

12

89

S

Figure 8. Acknowledge

0

1

1

A2

A1

A0

ACK

0

SDA

SCL

Figure 9. Slave Address